1. Field of the Invention
The present invention relates generally to borehole logging tools. More specifically, the present invention relates to apparatuses and methods for mounting transducers near a borehole wall.
2. Background Art
Borehole logging tools are used in the evaluation of formations surrounding boreholes for extraction of hydrocarbons. One important consideration in the design of logging tools is the coupling of transducers to the formation for deriving formation properties from formation logging data. Such coupling is needed to provide adequate signals in a downhole environment where power available to the transducers may be limited, such as in a downhole environment of a logging-while-drilling (LWD) system, which may be battery powered.
A typical LWD drilling assembly is illustrated in FIGS. 1-2. A drill string 12 is suspended within a borehole and includes a drill bit 15 at its lower end. A surface system includes a derrick assembly 2 positioned over the borehole, the assembly 2 including a rotary table 16, kelly 17, and rotary swivel 19. The drill string 12 is rotated by the rotary table 16, which engages the kelly 17 at the upper end of the drill string 12. The surface system further includes wellbore fluid 26 stored in a pit 27 formed at the well site. A pump 29 delivers the wellbore fluid 26 to the interior of the drill string 12 via a port in the swivel 19, causing the wellbore fluid to flow downwardly through the drill string 12, as indicated by a directional arrow 8. The wellbore fluid 26 exits the drill string 12 via ports in the drill bit 15, and then circulates upwardly through the annulus region between the outside of the drill string 12 and the wall of the borehole, as indicated by directional arrows 9. In this well-known manner, the wellbore fluid 26 lubricates the drill bit 15 and carries formation cuttings up to the surface as it is returned to the pit 27 for recirculation.
In order to derive formation properties from formation logging data, transducers or sensors are mounted onto a drilling assembly such as the one described above. In the LWD drilling assembly of FIG. 1, transducers 14 are mounted inside a drill collar 50. As shown in FIG. 14, sleeve 6 having low-density windows 6a, may be engaged to the drill collar 50 to collimate the passage of signals to and help exclude mud from the transducers 14. In FIG. 2, transducers 24 are mounted on a stabilizer 40 integrated with a body of the drill collar 50. The stabilizer 40 is an integral physical part of the drill collar 50.